Providing an inexpensive high-capacity bidirectional data link to user terminals in remote areas poses many challenges. Because user terminals in remote areas are typically distributed sparsely over a large geographic area (e.g. tens of terminals over hundreds of square kilometers), the cost of deploying a wired network is prohibitive. Wireless communication networks, with a point-to-multipoint topology comprising a network hub or access point with which multiple user terminals communicate independently and bidirectionally, are a more promising technology to deploy.
In digital broadcasting a video stream of 20 MBits/sec can be delivered from an access point to any number of user terminals over a radius of tens of kilometers within a 7 MHz bandwidth in the VHF frequency band. However, in a broadcasting application the data is unidirectional and common to all user terminals, so the required capacity to service all users is independent of the number of user terminals.
Candidate wireless technologies for independent bidirectional data transmission such as WiMAX (IEEE 802.16), which typically operates at a carrier frequency above 2 GHz, suffer from two related problems:    1. Inadequate coverage. The distance between an access point and a user terminal is limited to less than 10 kilometers at a carrier frequency above 2 GHz using an access point antenna height of less than 30 m in a point-to-multipoint topology.    2. Inadequate capacity. Current WiMAX technology typically provides a spectral efficiency of 2 to 5 bits/sec/Hz (i.e. 20 to 50 MBits/sec per 10 MHz frequency channel). This capacity needs to be shared among, potentially, thousands of users. To provide simultaneous access at data rates of 1 to 20 MBits/s to this number of users from a single access point requires a prohibitively large bandwidth at the carrier frequency.
There is a tradeoff between these two problems in that capacity can be sacrificed for coverage, or vice versa, by decreasing or increasing the carrier frequency respectively. A possible way out of the tradeoff is to increase the transmit power from the access point and the user terminals. This however increases the cost of the system.
A satisfactory compromise providing acceptable bidirectional data rates to all users in a sufficiently wide coverage area at low enough power levels to yield acceptable cost is yet to be found with WiMAX or other conventional technologies.